Carbon-13 relaxation behavior in multiply enriched systems: [90%-1,2-13C2-Gly6]Bradykinin

The relaxation behavior of macromolecules multiply enriched in 13C has been investigated theoretically, and results for [90%-1,2-13C2-Gly6] bradykinin have been interpreted on the basis of this theory. The 13C-13C dipolar interaction can be selectively observed by comparing the relaxation behavior of the singly labeled (center) and doubly labeled satellite 13C resonances, and the results can be used to evaluate the rotational correlation time of carbon-carbon bonds. In both the present study and all of the multiple labeling studies reported to date, the rotational correlation times thus obtained (tau/sub CC/) are significantly longer than the rotational correlation times (tau/sub CH/) obtained in the usual way on the basis of relaxation analysis of protonated carbons. The spin-lattice relaxation behavior for doubly 13C labeled system falls into three limits: (1) extreme narrowing limit in which the 13C-1H and 13C-13C dipolar interactions are additive; (2) spin-diffusion limit in which the relaxation rate is limited by the rate of 13C-13C flip-flop interactions leading to spin diffusion; (3) spin-diffusion limit in which the rate is determined by the protonated carbon which becomes a relaxation sink. In addition to the spin-lattice relaxation differences between singly and multiply labeled molecules, significant line-width differences are also observed